Surface Cross-Linked Superabsorbent Polymer Particles and Methods of Making Them

ABSTRACT

Superabsorbent polymer particles with improved surface cross-linking and their use in absorbent articles. The superabsorbent polymer particles comprise polymer chain segments, wherein at least a part of the polymer chain segments are covalently cross-linked to each other and wherein at least a part of the cross-links include the reaction product of cross-linking molecules having at least two C═C double bonds and further include the reaction product of radical former molecules. These cross-links are present at surfaces of the superabsorbent polymer particles.

FIELD OF THE INVENTION

The present invention relates to superabsorbent polymer particles withimproved surface cross-linking and their use in absorbent articles.

Moreover, the invention relates to a process for making thesesuperabsorbent polymer particles.

BACKGROUND OF THE INVENTION

Superabsorbent polymers (SAPs) are well known in the art. They arecommonly applied in absorbent articles, such as diapers, training pants,adult incontinence products and feminine care products to increase theabsorbent capacity of such products while reducing their overall bulk.The SAPs generally are capable of absorbent and retaining amounts ofaqueous fluids equivalent to many times their own weight.

Commercial production of SAPs began in Japan in 1978. The earlysuperabsorbent was a cross-linked starch-g-polyacrylat0065. Partiallyneutralized polyacrylic acid eventually replaced earlier superabsorbentsin the commercial production of SAPs, and is the primary polymeremployed for SAPs today. SAPs are often applied in form of smallparticles, such as fibers or granules. They generally consist of apartially neutralized lightly cross-linked polymer network, which ishydrophilic and permits swelling of the network once submerged in wateror an aqueous solution such as physiological saline. The cross-linksbetween the polymer chains assure that the SAP does not dissolve inwater.

After absorption of an aqueous solution, swollen SAP particles becomevery soft and deform easily. Upon deformation the void spaces betweenthe SAP particles are blocked, which drastically increases the flowresistance for liquids. This is generally referred to as “gel-blocking”.In gel blocking situations liquid can move through the swollen SAPparticles only by diffusion, which is much slower than flow in theinterstices between the SAP particles.

One commonly applied way to reduce gel blocking is to make the particlesstiffer, which enables the SAP particles to retain their original shapethus creating or maintaining void spaces between the particles. Awell-known method to increase stiffness is to cross-link the carboxylgroups exposed on the surface of the SAP particles. This method iscommonly referred to as surface cross-linking.

The art refers e.g. to surface cross-linked and surfactant coatedabsorbent resin particles and a method of their preparation. The surfacecross-linking agent can be a polyhydroxyl compound comprising at leasttwo hydroxyl groups, which react with the carboxyl groups on the surfaceof the SAP particles. In some art, surface cross-linking is carried outat temperatures of 150° C. or above. The particles are preferablyexposed to the elevated temperatures for at least 5 minutes but lessthan 60 minutes.

Another known method for surface cross-linking absorbent resins uses thecarboxyl groups of the polymer, which are comprised on the surface ofthe resin, react with a polyhydric alcohol. The reaction can beaccomplished at temperatures in the range of 90° C. to 250° C.

It is also know that hydroxyalkylurea or hydroxyalkylamide can be usedas cross-linking agent. In both cases the surface cross-linking reactioncan be carried out at temperatures from about 90° C. to about 170° C.for 60 to 180 minutes.

A water-soluble peroxide radical initiator as surface cross-linkingagent is also known. An aqueous solution containing the surfacecross-linking agent is applied on the surface of the polymer. Thesurface cross-linking reaction is achieved by heating to a temperaturesuch that the peroxide radical initiator is decomposed while the polymeris not decomposed.

More recently the use of an oxetane compound and/or an imidazolidinonecompound for use as surface cross-linking agent has been disclosed. Thesurface cross-linking reaction can be carried out under heat, whereinthe temperature is preferably in the range of 60° C. to 250° C.Alternatively, the surface cross-linking reaction can also be achievedby a photo-irradiation treatment, preferably using ultraviolet rays.

In general, the surface cross-linking agent is applied on the surface ofthe SAP particles. Therefore, the reaction preferably takes place on thesurface of the SAP particles, which results in improved cross-linking onthe surface of the particles while not substantially affecting the coreof the particles. Hence, the SAP particles become stiffer andgel-blocking is reduced.

A drawback of the commercial surface cross-linking process describedabove is, that it takes relatively long, commonly at least about 30 min.However, the more time is required for the surface cross-linkingprocess, the more surface cross-linking agent will penetrate into theSAP particles, resulting in increased cross-linking inside theparticles, which has a negative impact on the capacity of the SAPparticles. Therefore, it is desirable to have short process times forsurface cross-linking. Furthermore, short process times are alsodesirable with respect to an overall economic SAP particle manufacturingprocess.

Another drawback of common surface cross-linking processes is, that theytake place only under relatively high temperatures, often around 150° C.or above. At these temperatures, not only the surface cross-linkerreacts with the carboxyl groups of the polymer, but also other reactionsare activated, e.g. anhydride-formation of neighbored carboxyl groupswithin or between the polymer chains, and dimer cleavage of acrylic aciddimers incorporated in the SAP particles. Those side reactions alsoaffect the core, decreasing the capacity of the SAP particles. Inaddition, exposure to elevated temperatures can lead to colordegradation of the SAP particles. Therefore, these side reactions aregenerally undesirable.

SAPs known in the art are typically partially neutralized, e.g. withsodium hydroxide. However, neutralization has to be carefully balancedwith the need for surface cross-linking: The surface cross-linkingagents known in the art only react with free carboxyl groups comprisedby the polymer chains but they are not able to react with a neutralizedcarboxyl groups. Thus, the carboxyl groups can either be applied forsurface cross-linking or for neutralization, but the same carboxyl groupcannot be applied to fulfill both tasks. Surface cross-linking agentsknown in the art do not react with chemical groups other than carboxylgroups, e.g. they do not react with aliphatic groups.

In the process of making SAP particles, neutralization of free carboxylgroups typically comes first, before surface cross-linking takes place.Indeed, the neutralization step is often carried out in the verybeginning of the process, before the monomers are polymerized andcross-linked to form the SAP. Such a process is named‘pre-neutralization process’. Alternatively, the SAP can be neutralizedin the middle of polymerization or after polymerization(‘post-neutralization’). Furthermore, a combination of thesealternatives is also possible.

As the overall number of free carboxyl groups on the outer surface ofthe SAP particles is limited by the foregoing neutralization, it is verydifficult to obtain particles with a high degree of surfacecross-linking and hence, a high stiffness to reduce gel-blocking.Furthermore, it is very difficult to obtain SAP particles with evenlydistributed surface cross-linking, as the remaining free carboxyl groupsare not only few in number but generally also randomly distributed,which sometimes results in SAP particles with regions of rather densesurface cross-linking and regions of sparsely surface cross-linking.

It is therefore an objective of the present invention to provide SAPparticles, which have a high degree of surface cross-linking and at thesame time allow for a high degree of neutralization.

It is a further objective of the present invention to provide SAPparticles with evenly distributed, homogenous surface cross-linking.

Furthermore, it is an objective of the present invention to provide aprocess to produce SAP particles with the above-mentioned advantages.

It is a still further objective of the present invention to provide aprocess to produce SAP particles, wherein the process step of surfacecross-linking can be carried out quickly to increase the efficiency ofthe process.

Moreover, a further objective of the present invention is to provide aprocess to produce SAP particles, which can be carried out at moderatetemperatures in order to reduce undesired side reactions, such asanhydride-formation and dimer cleavage.

SUMMARY OF THE INVENTION

The present invention relates to superabsorbent polymer particlescomprising polymer chain segments,

-   -   wherein at least some of the polymer chain segments are        covalently cross-linked to each other after formation of the        superabsorbent polymer particles, and    -   wherein the cross-links comprise the reaction product of        cross-linking molecules having at least two C═C double bonds,        and wherein the cross-links further comprise the reaction        product of radical former molecules, and    -   wherein the cross-links are present on the surface of the        superabsorbent polymer particles.

The present invention further relates to a method of surfacecross-linking superabsorbent polymer particles which comprises the stepsof

a) providing superabsorbent polymer particles comprising polymer chainsegments,

b) adding a surface cross-linking composition comprising cross-linkingmolecules having at least two C═C double bonds and further comprisingradical former molecules,

c) exposing the superabsorbent polymer particles and the surfacecross-linking composition to electromagnetic irradiation capable ofactivating the radical former,

whereby the cross-linking molecules and the radical former moleculesreact with at least some of the polymer chain segments comprised atsurfaces of the superabsorbent polymer particles to form covalentcross-links between the polymer chain segments, wherein the cross-linkscomprise the reaction product of the cross-linking molecule and whereinthe cross-links further comprise the reaction product of the radicalformer molecules.

Moreover, the present invention relates to absorbent products comprisingthe superabsorbent polymer particles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims pointing out anddistinctly claiming the present invention, it is believed the same willbe better understood by the following drawings taken in conjunction withthe accompanying specification wherein like components are given thesame reference number.

FIG. 1 is a top plan view of a disposable diaper, with the upper layerspartially cut away.

FIG. 2 is a cross-sectional view of the disposable diaper shown in FIG.1

DETAILED DESCRIPTION OF THE INVENTION

The SAPs according to the present invention comprise a homopolymer ofpartially neutralized α,β-unsaturated carboxylic acid or a copolymer ofpartially neutralized α,β-unsaturated carboxylic acid copolymerized witha monomer copolymerizable therewith. Furthermore, the homo-polymer orcopolymer comprised by the SAP comprises aliphatic groups, wherein atleast some of the aliphatic groups are at least partially exposed on thesurface of the superabsorbent polymer particles

SAPs are available in a variety of chemical forms, including substitutedand unsubstituted natural and synthetic polymers, such as carboxymethylstarch, carboxymethyl cellulose, and hydroxypropyl cellulose; nonionictypes such as polyvinyl alcohol, and polyvinyl ethers; cationic typessuch as polyvinyl pyridine, polyvinyl morpholinione, and N,N-dimethylaminoethyl or N,N-diethylaminopropyl acrylates andmethacrylates, and the respective quaternary salts thereof. Typically,SAPs useful herein have a multiplicity of anionic, functional groups,such as sulfonic acid, and more typically carboxyl groups. Examples ofpolymers suitable for use herein include those, which are prepared frompolymerizable, unsaturated, acid-containing monomers. Thus, suchmonomers include the olefinically unsaturated acids and anhydrides thatcontain at least one carbon-to-carbon olefinic double bond. Morespecifically, these monomers can be selected from olefinicallyunsaturated carboxylic acids and acid anhydrides, olefinicallyunsaturated sulfonic acids, and mixtures thereof.

Some non-acid monomers can also be included, usually in minor amounts,in preparing SAPs. Such non-acid monomers can include, for example, thewater-soluble or water-dispersible esters of the acid-containingmonomers, as well as monomers that contain no carboxylic or sulfonicacid groups at all. Optional non-acid monomers can thus include monomerscontaining the following types of functional groups: carboxylic acid orsulfonic acid esters, hydroxyl groups, amide-groups, amino groups,nitrile groups, quaternary ammonium salt groups, aryl groups (e.g.,phenyl groups, such as those derived from styrene monomer). Thesenon-acid monomers are well-known materials and are described in greaterdetail, for example, in U.S. Pat. No. 4,076,663 and in U.S. Pat. No.4,062,817.

Olefinically unsaturated carboxylic acid and carboxylic acid anhydridemonomers include the acrylic acids typified by acrylic acid itself,methacrylic acid, ethacrylic acid, α-chloroacrylic acid, α-cyanoacrylicacid, β-methylacrylic acid (crotonic acid), α-phenylacrylic acid,β-acryloxypropionic acid, sorbic acid, α-chlorosorbic acid, angelicacid, cinnamic acid, p-chlorocinnamic acid, β-sterylacrylic acid,itaconic acid, citroconic acid, mesaconic acid, glutaconic acid,aconitic acid, maleic acid, fumaric acid, tricarboxyethylene and maleicacid anhydride.

Olefinically unsaturated sulfonic acid monomers include aliphatic oraromatic vinyl sulfonic acids such as vinylsulfonic acid, allyl sulfonicacid, vinyl toluene sulfonic acid and styrene sulfonic acid; acrylic andmethacrylic sulfonic acid such as sulfoethyl acrylate, sulfoethylmethacrylate, sulfopropyl acrylate, sulfopropyl methacrylate,2-hydroxy-3-methacryloxypropyl sulfonic acid and2-acrylamide-2-methylpropane sulfonic acid.

Preferred SAPs according to the present invention contain carboxylgroups. These polymers comprise hydrolyzed starch-acrylonitrile graftcopolymers, partially neutralized hydrolyzed starch-acrylonitrile graftcopolymers, starch-acrylic acid graft copolymers, partially neutralizedstarch-acrylic acid graft copolymers, saponified vinyl acetate-acrylicester copolymers, hydrolyzed acrylonitrile or acrylamide copolymers,slightly network crosslinked polymers of any of the foregoingcopolymers, partially neutralized polyacrylic acid, and slightly networkcrosslinked polymers of partially neutralized polyacrylic acid,partially neutralized polymethacrylic acid, and slightly networkcrosslinked polymers of partially neutralized polymethacrylic acid.These polymers can be used either solely or in the form of a mixture oftwo or more different polymers, that when used as mixtures, individuallydo not have to be partially neutralized, whereas the resulting copolymerhas to be. Examples of these polymer materials are disclosed in U.S.Pat. No. 3,661,875, U.S. Pat. No. 4,076,663, U.S. Pat. No. 4,093,776,U.S. Pat. No. 4,666,983, and U.S. Pat. No. 4,734,478.

Most preferred polymer materials for use herein are slightly networkcrosslinked polymers of partially neutralized polyacrylic acids,slightly network crosslinked polymers of partially neutralizedpolymethacrylic acids, their copolymers and starch derivatives thereofMost preferably, SAPs comprise partially neutralized, slightly networkcrosslinked, polyacrylic acid (i.e. poly (sodium acrylate/acrylicacid)). Preferably, the SAPs are at least 50%, more preferably at least70%, even more preferably at least 75% and even more preferably from 75%to 95% neutralized. Network crosslinking renders the polymersubstantially water-insoluble and, in part, determines the absorptivecapacity of the hydrogel-forming absorbent polymers. Processes fornetwork crosslinking these polymers and typical network crosslinkingagents are described in greater detail in U.S. Pat. No. 4,076,663.

A suitable method for polymerizing the α,β-unsaturated carboxylic acidmonomers is aqueous solution polymerization, which is well known in theart. An aqueous solution comprising α,β-unsaturated carboxylic acidmonomers and polymerization initiator is subjected to a polymerizationreaction. The aqueous solution may also comprise further monomers, whichare co-polymerizable with the α,β-unsaturated carboxylic acid monomers.At least the α,β-unsaturated carboxylic acid has to be partiallyneutralized, either prior to polymerization of the monomers, duringpolymerization or post polymerization. In a preferred embodiment of thepresent invention, the monomers (including α,β-unsaturated carboxylicacid monomers and possible comonomers) are at least 50 %, morepreferably at least 70%, even more preferably at least 75% and even morepreferably from 75% to 95% neutralized.

The monomers in aqueous solution are polymerized by standard freeradical techniques, commonly by using a photoinitiator for activation,such as ultraviolet (UV) light. Alternatively, a redox initiator may beused. In this case, however, increased temperatures are necessary.

The water-absorbent resin will preferably be lightly cross-linked torender it water-insoluble. The desired cross-linked structure may beobtained by the co-polymerization of the selected water-soluble monomerand a cross-linking agent possessing at least two polymerizable doublebonds in the molecular unit. The cross-linking agent is present in anamount effective to cross-link the water-soluble polymer. The preferredamount of cross-linking agent is determined by the desired degree ofabsorption capacity and the desired strength to retain the absorbedfluid, that is, the desired absorption under load. Typically, thecross-linking agent is used in amounts ranging from 0.0005 to 5 parts byweight per 100 parts by weight of monomers (including α,β-unsaturatedcarboxylic acid monomers and possible comonomers) used. If an amountover 5 parts by weight of cross-linking agent per 100 parts is used, theresulting polymer has a too high cross-linking density and exhibitsreduced absorption capacity and increased strength to retain theabsorbed fluid. If the cross-linking agent is used in an amount lessthan 0.0005 parts by weight per 100 parts, the polymer has a too lowcross-linking density and when contacted with the fluid to be absorbedbecomes rather sticky, water-soluble and exhibits a low absorptionperformance, particularly under load. The cross-linking agent willtypically be soluble in the aqueous solution.

Alternatively to co-polymerizing the cross-linking agent with themonomers, it is also possible to cross-link the polymer chains in aseparate process step after polymerization.

After polymerization, cross-linking and partial neutralization, theviscous SAPs are dehydrated (i.e. dried) to obtain dry SAPs. Thedehydration step can be performed by heating the viscous SAPs to atemperature of about 120° C. for about 1 or 2 hours in a forced-air ovenor by heating the viscous SAPs overnight at a temperature of about 60°C. The content of residual water in the dehydrated SAP after dryingpredominantly depends on dyring time and temperature and can range from0.5% by weight of dry SAP up to 50% by weight of dry SAP. Preferably,the content of residual water in the dehydrated SAP after drying is 0.5%-45% by weight of dry SAP, more preferably 0.5% -30%, even morepreferred 0.5% -15% and most preferred 0.5% -5%.

The SAPs can be transferred into particles of numerous shapes. The term“particles” refers to granules, fibers, flakes, spheres, powders,platelets and other shapes and forms known to persons skilled in the artof SAPs. E.g. the particles can be in the form of granules or beads,having a particle size of about 10 to 1000 μm, preferably about 100 to1000 μm. In another embodiment, the SAPs can be in the shape of fibers,i.e. elongated, acicular SAP particles. In those embodiments, the SAPfibers have a minor dimension (i.e. diameter of the fiber) of less thanabout 1 mm, usually less than about 500 μm, and preferably less than 250μm down to 50 μm. The length of the fibers is preferably about 3 mm toabout 100 mm. The fibers can also be in the form of a long filament thatcan be woven.

According to the present invention the dehydrated SAP particles undergoa surface cross-linking process step. The term “surface” describes theouter-facing boundaries of the particle. For porous SAP particles,exposed internal surfaces may also belong to the surface. The term“surface cross-linked SAP particle” refers to an SAP particle having itsmolecular chains present in the vicinity of the particle surfacecross-linked by a compound referred to as surface cross-linker. Thesurface cross-linker is applied to the surface of the particle. In asurface cross-linked SAP particle the level of cross-links in thevicinity of the surface of the SAP particle is generally higher than thelevel of cross-links in the interior of the SAP particle.

Commonly applied surface cross-linkers are thermally activatable surfacecross-linkers. The term “thermally activatable surface cross-linkers”refers to surface cross-linkers, which only react upon exposure toincreased temperatures, typically around 150° C. Thermally activatablesurface cross-linkers known in the prior art are e.g. di- orpolyfunctional agents that are capable of building additionalcross-links between the polymer chains of the SAPs. Other thermallyactivatable surface cross-linkers include, e.g., di- or polyhydricalcohols, or derivatives thereof, capable of forming di- or polyhydricalcohols. Representatives of such agents are alkylene carbonates,ketales, and di- or polyglycidlyethers. Moreover, (poly)glycidyl ethers,haloepoxy compounds, polyaldehydes, polyoles and polyamines are alsowell known thermally activatable surface cross-linkers. Thecross-linking is based on a reaction between the functional groupscomprised by the polymer, for example, an esterification reactionbetween a carboxyl group (comprised by the polymer) and a hydroxyl group(comprised by the surface cross-linker). As typically a relatively bigpart of the carboxyl groups of the polymer chain is neutralized prior tothe polymerization step, commonly only few carboxyl groups are availablefor this surface cross-linking process known in the art. E.g. in a 70%percent neutralized polymer only 3 out of 10 carboxylic groups areavailable for covalent surface cross-linking.

The method of the present invention is applied for surface cross-linkingof SAP particles. Hence, the polymer chains comprised by the SAPparticles commonly already have been cross-linked by a cross-linkerknown in the art, comprising at least two polymerizable double bonds inthe molecule unit. The cross-linking of different polymer chain segmentsof the present invention is not intended to bond different SAP particlesto each other. Thus, the method of the present invention does not leadto any appreciable inter-particulate bonds between different SAPparticles but only results in intra-particulate direct covalent bondswithin an SAP particle. If present, such interparticulate directcovalent bonds would hence require additional inter-particulatecross-linking materials.

For the present invention, wherein the polymer chains have already beencross-linked and are thus provided in form of a network, the term“polymer chain segment” refers to the part of the polymer chains betweentwo neighboring, existing cross-links or to the part of the polymerchains between sites, where the polymer chain is branched.

Cross-Linking Molecules

The cross-linking molecules of the present invention comprise at leasttwo C═C double bonds. Preferably, the cross-linking molecules comprisemore than two C═C double bonds.

Preferred cross-linking molecules of the present invention arepolyfunctional allyl and acryl compounds, such as triallyl cyanurate,triallyl isocyanurate, trimethylpropane tricrylate or other triacrylateesters, pentaerythritol triallyl ether, pentaerythritol tetraallylether, butanediol diacrylate, pentaerythritol tetraacrylate, tetraallylorthosilicate, di-pentaerythritol pentaacyralate,di-pentaerythritol hexaacyralate, ethyleneglycol diacrylate,ethyleneglycol dimethacrylate, tetra allyloxy ethane, diallyl phthalate,diethyleneglycol diacrylate, allylmethacrylate, triallylamine,1,1,1-trimethylolpropane triacrylate, triallyl citrate, or triallylamine.

Alternatively, the cross-linking molecules are selected from the groupconsisting of squalene, N,N′ methylenebisacrylamide, icosa-pentaenicacid, or sorbic acid.

The most preferred cross-linking molecule of the present invention istriallyl cyanurate.

Radiation activatable radical former molecules

The radiation activatable radical former molecules are able to formradicals upon electromagnetic radiation.

According to the present invention, the radical former molecules canbelong to two different types of radical former: a) Radical formermolecules undergoing photo-fragmentation upon irradiation and b) radicalformer molecules undergoing photo-reduction upon irradiation. Thereaction mechanism of both types is described in detail below. Accordingto the present invention, it is not preferred to mix radical formermolecules of type a) with radical former molecules of type b).

Radical former molecules of type a) can be selected from the groupconsisting of dialkyl peroxy-dicarbonates, benzilketales, di-tert-butylperoxide, di-benzoyl peroxide, bis-(aroyloxyl) peroxides such asbis-(4-methoxy) di-benzoyl peroxide, or bis-(4-methyl) di-benzoylperoxide, or bis-(4-chlor) di-benzoyl peroxide, 2,4,6-tri-methyldi-benzoyl peroxide, 3-benzoyl benzoic acid, 1,3-dibenzoyl propane,di-benzoyl disulphide, S-phenyl thiobenzoates, acylphosphine oxides,benzoylphosphineoxides, aryl-aryl-sulphides, di-benzoyl methanes,phenylazo-di-phenyl sulphone, substituted dialkyl peroxy-dicarbonates,substituted benzilketales, substituted di-tert-butyl peroxides,substituted di-benzoyl peroxides, substituted bis-(aroyloxyl) peroxidessuch as substituted bis-(4-methoxy) di-benzoyl peroxide, or substitutedbis-(4-methyl) di-benzoyl peroxide, or substituted bis-(4-chloro)di-benzoyl peroxide, substituted 2,4,6-tri-methyl di-benzoyl peroxide,substituted 3-benzoyl benzoic acid, substituted 1,3-dibenzoyl propane,substituted O-acyl α-oximinoketones, substituted di-benzoyl disulphide,substituted S-phenyl thiobenzoates, substituted acylphosphine oxides,substituted benzoylphosphineoxides, substituted aryl-aryl-sulphides,substituted di-benzoyl methanes, substituted phenylazo-di-phenylsulphone, the cyclic peroxide of phthalic acid and its derivatives, andthe cyclic peroxides of succinic acid and its derivatives. In apreferred embodiment of the invention, such derivatization is done toeither enable or further enhance water-solubility.

Radical formers of type b) can be selected from the group consisting ofof acetophenone, benzophenone, anthraquinone, xanthone, thioxanthone,camphorquinone, terephthalophenone, benzil, fluorenone, α-ketocoumarinas well as acetophenone-, benzophenone-, anthraquinone-, xanthone-,thioxanthone-, camphorquinone-, terephthalophenone-, benzil-,fluorenone-, α-ketocoumarin-derivatives. Suitable acetophenonederivatives or benzophenone derivatives, for example, also comprisereaction products, such as condensation products of acetophenonederivatives or benzophenone derivatives, comprising at least twoacetophenone or benzophenone groups. In a preferred embodiment of theinvention, such derivates are chosen to enable or further enhancewater-solubility of the radical former molecule.

Alternatively, the radical former molecules of type b) comprise a firstgroup selected from the group consisting of methyl, benzyl, aryl,preferably phenyl and substituted phenyl, and a second group selectedfrom the group consisting of an aryl, an alkyl of 1 to 4 carbon atoms,cyclopropyl, cyclopentyl, cyclohexyl, α,α-dialkoxyalkyl, andα-hydroxyalkyl and wherein the first group is covalently bound to thesecond group via an additional carbonyl group.

Preferred radical former molecules according to the present inventionhave a molecular weight of at least Mw=25 g/mol, more preferred at leastMw=60 g/mol, still more preferred at least Mw=120 g/mol, even morepreferred at least Mw=180 g/mol and most preferred at least Mw=240g/mol. Radical former molecules having a relatively high molecularweight often tend to form more stable radicals, as the charge of theradical can be distributed better within the radical. Without wishing tobe bound be theory, it is believed that if the radical were veryunstable, it more likely reacts to recombine to the radical formermolecule.

Furthermore, preferred radical former molecules according to the presentinvention will comprise aromatic groups, such as arenes. This also leadsto more stable radicals as the unpaired electron can be distributedthroughout the aromatic group.

Particularly preferred radical former molecules of the present inventionare acetophenone- or benzophenone-derivatives.

Suitable acetophenone derivatives or benzophenone derivatives aredescribed, for example, in European Patent Application EP-A-0 346 734;European Patent Application EP-A-0 377 199; European Patent ApplicationEP-A-0 246 848; German Patent Application DE-A-4 037 079 and GermanPatent Application DE-A-3 844 444.

Reaction Mechanism:

In the following, the principal reaction mechanism according to thepresent invention is depicted.

a) Radical Former Molecules Undergoing Photo-Fragmentation UponIrradiation

The radical former molecule of this type comprises a labile bond, and ishereinafter generally depicted as R_(a)—R_(b). Upon electromagneticirradiation, preferably UV radiation, the labile bond breaks, wherebytwo radicals (R_(a) and R_(b)) are formed according to Formula 1. Thishomolytic cleavage may result in two identical radicals, if the labilebond comprised by the radical former molecule (so-called precursormolecule) divides the molecule into two identical parts. Alternatively,the homolytic cleavage may result in two different radicals.

Formula 1:

The radicals, which have been formed, can now react with an aliphaticC—H group comprised by a polymer chain segment of the SAP particleforming a carbon-centered radical at this polymer chain segmentaccording to Formula 2. Alternatively, the radicals formed from theradical former molecule, can react with one of the C═C double bondscomprised by the cross-linking molecule to form a radical consisting ofthe reaction product of the cross-linking molecule and the initialradical according to Formula 3.

The carbon-centered radical within the polymer chain segment formed inthe reaction of Formula 2 can react with the radical formed in Formula3. The reaction product of this reaction is a polymer chain segment,which has the reaction products of the radical former molecule and thecross-linking molecule bound thereto according to Formula 4.

Thereafter, the radicals formed from the radical former molecule inFormula 1, can react with the second of the C═C double bonds of thecross-linking molecule, which is comprised in the reaction product ofFormula 4. This reaction is depicted in Formula 5:

To form the cross-link between two polymer chain segments, thecarbon-centered radical which is comprised in the reaction product ofFormula 5 combines with another carbon centered radical located at apolymer chain segment, which forms as described by Formula 2, closingthe cross-link. This reaction is depicted in Formula 6.

Hence, the net reaction when using radical former molecules undergoingphoto-fragmentation upon irradiation is the formation of a cross-linkbetween two polymer chain segments, wherein the cross-link comprises thereaction product of one cross-linking molecule with two C═C double bondsand two radical former molecules. The net reaction is depicted in

Formula 7:

In case of symmetric radical former molecules which form two identicalradicals, it is possible, to recycle the resulting R_(a)—H and/orR_(b)—H molecules in order to regain the initial radical formermolecules.

In the above described reaction mechanism, side reactions maytheoretically take place, such as:

-   -   Recombination of two radicals formed upon homolytic cleavage of        the radical former molecule. However, the recombined radical        former molecule may again form radicals upon electromagnetic        irradiation, or    -   Two carbon-centered radicals formed at different polymer chain        segments according to Formula 2 may combine to form a direct        covalent bond between these polymer chain segments. As this side        reaction also leads to the formation of a cross-link between two        polymer chain segments, this side reaction does not have any        negative impact on the present invention.        b) Radical Former Molecules Undergoing Photo-Reduction upon        Irradiation

Radical former molecules undergoing photo-reduction upon irradiationcomprise carbonyl groups. In preferred embodiments of the presentinvention, such radical former molecules are ketones.

Upon electromagnetic irradiation, preferably upon irradiation by UVlight, the radical former molecules of this type are transferred in an“excited state” (triplet state) according to Formula 8. Hence, they arenot yet transformed into a radical, but are much more reactive thanbefore they were irradiated.

In the next step, the radical former molecule in its excited statereacts with an aliphatic C—H group comprised by a polymer chain segmentand abstracts a hydrogen radical, thereby forming a carbon-centeredradical at this polymer chain segment and a ketyl radical according toFormula 9:

The ketyl radical can now react with one of the C═C double bonds of thecross-linking molecule, whereby the ketyl radical reacts with thecross-linking molecule (addition reaction to the C═C double bond), thusforming another radical according to Formula 10:

The carbon-centered radical within the polymer chain segment formed inthe reaction of Formula 8 can now react with the radical formed inFormula 10. The reaction product of this reaction is a polymer chainsegment, which has the reaction products of the radical former moleculeand the cross-linking molecule bound thereto according to Formula 11.

Thereafter, the ketyl radical formed from the radical former molecule inFormula 9, can react with the second of the C═C double bonds of thecross-linking molecule, which is comprised in the reaction product ofFormula 11. This reaction is depicted in Formula 12:

To form the cross-link between two polymer chain segments, thecarbon-centered radical which is comprised in the reaction product ofFormula 12 combines with another carbon centered radical located at apolymer chain segment, which forms as described by Formula 9, closingthe cross-link. This reaction is depicted in Formula 13.

Hence, the net reaction when using radical former molecules undergoingphoto-reduction upon irradiation is the formation of a cross-linkbetween two polymer chain segments, wherein the cross-link comprises thereaction product of one cross-linking molecule with two C═C double bondsand two radical former molecules. The net reaction is depicted inFormula 14:

In the above described reaction mechanism, side reactions maytheoretically take place, such as:

-   -   Combination of two ketyl radicals formed upon homolytic cleavage        of the radical former molecule, or    -   Two carbon-centered radicals formed at different polymer chain        segments according to Formula 2 may combine to form a direct        covalent bond between these polymer chain segments. As this side        reaction also leads to the formation of a cross-link between two        polymer chain segments, this side reaction does not have any        negative impact on the present invention.

According to the present invention, radical former molecules undergoingphoto-reduction upon irradiation are preferred over radical formermolecules undergoing photo-fragmentation.

It should be noted, that in the case of radical former moleculesundergoing photo-fragmentation are applied, only a part of the radicalformer molecule is comprised by the cross-link between the polymer chainsegments, whereas for radical former molecules undergoingphoto-reduction, the complete radical former molecule in its reducedform (with a carbonyl group being reduced to a hydroxyl group) iscomprised by the cross-link between the polymer chain segments.

Hence, for radical former molecules undergoing photo-fragmentation, thereaction product comprised by the cross-link between polymer chainsegments is only a part of the initial radical former molecule—typicallyone half of the initial molecule.

For radical former molecules undergoing photo-reduction, the reactionproduct comprised by the cross-link between polymer chain segments isthe complete radical former molecule in its reduced form (with acarbonyl group being reduced to a hydroxyl group).

The reaction product of the cross-linking molecule -for both types ofradical former molecules- is the initial cross-linking molecule, whereinthose C═C double bonds, which have reacted with the radicals formed fromthe radical former molecules (or have reacted directly with thecarbon-centered radicals formed in the polymer chain segments) areconverted into C—C single bonds.

As the reaction substantially only takes place at the surface of the SAPparticles, the cross-links between polymer chain segments according tothe present invention are mainly present on the surface of the SAPparticles, although a few of those cross-links may also be formed insidethe SAP particles. This is because a small amount of the surfacecross-linking composition may penetrate inside the SAP particles afterthe composition has been applied onto the SAP particle surfaces.However, these cross-links between polymer chain segments inside the SAPparticles are not an objective of the present invention but only maytake place unavoidably to a very small degree.

In preferred embodiments of the present invention -for both types ofradical former molecules- the cross-linking molecules comprise more thantwo C═C double bonds. In these embodiments, more than two polymer chainsegments can be cross-linked to each other, following the reactionmechanism described above. In these embodiments, the number of reactionproducts of radical former molecules comprised by the cross-link equalsthe number of C═C double bonds comprised by the cross-linking molecule.

It is believed, that in embodiments, wherein more than two polymer chainsegments are cross-linked to each other, the efficiency of the reactionas well as the stability of the resulting product is significantlyenhanced.

Without wanting to be bound by theory, it is believed that the ratedetermining step of a radically initiated cross-linking reaction in theabsence the cross-linking molecule is the so-called recombination of twocarbon centered radicals, forming a direct covalent bond between twopolymer chain segments. This recombination follows a kinetic law of asecond order reaction, i.e. the reaction rate of such combinationreaction is proportional to the product of the concentrations of the twocombining carbon centered radical species. To form a new covalent bond,the two carbon centered radicals need to hit each other during theirshort lifetime, despite their distance and possible stericalobstructions due to the rigidity of the polymer chains to be connected.

If, however, a cross-linking molecule is added according to the presentinvention, it is believed, that the reaction between the cross-linkingmolecule or its reaction product—i.e. the radical formed according toFormula 3 for type a) radical formers, respectively, the radical formedaccording to Formula 11 for type b) radical formers—and the carboncentered radical in the polymer chain, forming a covalent bond, followsa kinetic law of pseudo-first order, i.e. the reaction rate is believedto be only proportional to the concentration of the carbon centeredradical, since the concentration of the second reaction partner, i.e.the cross-linking molecule respectively its reaction product, is so highthat it can be regarded as constant throughout the reaction. Reactionsof pseudo-first order kinetics are known to be kinetically favoredversus reactions of second order kinetics, i.e. they have a higherreaction speed, in particular if the reactive species, in this case theintermediate carbon centered radical in the polymer chain, are low inconcentration. As a result thereof, the overall process can be run at ahigher line speed due to the presence of the surface cross-linker andits kinetically favorable influence on the rate-determining step of theoverall reaction.

As another consequence thereof, the overall process is more robusttowards the presence of oxygen due to the presence of thesurface-cross-linker. Oxygen is known as a radical-scavenger thatreadily reacts with carbon-centered radicals. If the desired reaction ofthe carbon-centered radicals is accelerated, as it is believed in thecase of the present invention, without wanting to be bound by theory,the unwanted side reaction with oxygen can be at least partlycircumvented. Hence, the necessary process measures to suppress presenceof oxygen during the reaction do not need to be exerted as rigorously,which may facilitate processing and decrease capital cost.

The SAP particles of the present invention can be analyzed by 13C—NMR or1H—NMR methods well known in the art to detect the reaction product ofthe cross-linking molecules having at least two C═C double bonds and ofthe radical former molecules. Process:

Above-mentioned radiation-activateable surface cross-linkingcompositions are capable of forming covalent bonds by exposure toelectromagnetic radiation. Electron beams as well as UV-light canproduce suitable electromagnetic radiation. Preferably, according to thepresent invention UV-light is used with a wave-length of 220-380 nm,selected on the selected radical former molecule(s). The UV-light may beused in combination with an electron-beam, and also in combination withan IR-light. In case of combination of UV-irradiation with otherelectromagnetic radiation, it is not critical if the application of theUV-light takes place simultaneously with the other electromagneticirradiation (i.e. electron-beam or IR-light), or if irradiation is donein a series of different irradiation steps. For radical formermolecules, which require a relative high amount of activation energy,activation with electron beams may be necessary.

In the present invention the surface cross-linking composition isapplied in amounts of less than 25% by weight of SAP particle,preferably in amounts of less than 15%, more preferably in amounts ofless than 5%, even more preferably in amounts from 0.1% to 5% and mostpreferably in amounts from 0.1% to 1.5%.

The ratio of the cross-linking molecule to the radical former moleculeis preferably in the range of 0.2 to 5, even more preferred between 0.33and 3 and mostly preferred in the range of 1 to 3, with said ratiosbeing molar ratios.

The surface cross-linking composition may be sprayed onto the SAPparticles by means of a fluidized-bed spraying chamber. SimultaneouslyIR-irradiation may be applied to accomplish drying and simultaneouslyUV-light may be applied to accomplish cross-linking in thefluidized-bed.

However, in certain cases drying and cross-linking may take place in twosteps in series, which could be carried out in any order. Instead or incombination with IR-light, any conventional drying equipment can be usedin the drying step. However, in certain embodiments of the presentinvention little or no drying is required, e.g. in cases, where only asmall amount of the surface cross-linking composition is applieddissolved in small amounts of solution.

In particular, the radiation activatable radical former molecules mayupon activation by irradiation react with aliphatic C—H bond comprisedby a nearby polymer chain segment, abstracting a hydrogen radical andleaving a carbon centered radical. Theoretically, the radiationactivatable radical former molecules may upon radiation also react withcarboxyl groups comprised by the polymer chain segments. However, it ismuch more likely that the radical formed from the radical formermolecule will react with the aliphatic C—H bond, as it is ratherunlikely that the radical will be able to abstract a hydrogen radicalfrom the carboxyl group, which is strongly polarized.

Hence, compared to prior art surface cross-linking, the cross-linkingprocess of the present invention is not restricted to the carboxylgroups but also comprises the numerous aliphatic groups within thepolymer chains of the SAP. Hence, according to the present invention thenumber of available reaction sites for the surface cross-linking processof the SAP particles is strongly increased. Therefore, it is possible toachieve a far more homogenous, uniform surface cross-linking compared tothe surface cross-linking known from the art. Furthermore, it ispossible to surface cross-link the SAP to a higher degree than the SAPknown from the prior art. This enables to make the SAP particles muchstiffer, thus, to more effectively inhibit the gel-blocking effect at agiven degree of neutralization. Moreover, it is possible to increase thecapacity of the SAP particles.

As the surface cross-linking composition is applied on the surface ofthe SAP particles, the reaction takes mainly place on the surface of theSAP particles. That means, that mainly aliphatic groups, which areexposed in the vicinity of the surface of the SAP particles, undergo across-linking process, leading to SAP particles with a high degree ofcross-linking on their surface while not substantially affecting theinner core (=interior portion) of the SAP particles. Hence, thepercentage of the reaction product of the radiation activatable radicalformer molecules and the cross-linking molecules on the surface of theSAP particles will preferably be higher than the percentage of saidreaction product inside the SAP particles.

The UV irradiation for the surface cross-linking can preferably becarried out in a conventional manner with UV lamps having a powerbetween 50 W and 2 kW, more preferably between 200 W and 700 W, and evenmore preferred between 400 W and 600 W. Irradiation time is preferablybetween 0.1 sec. and 30 min., more preferably between 0.1 sec. and 15min, even more preferably between 0.1 sec. and 5 min and most preferablybetween 0.1 sec. and 2 min. Commercially available mercury pressureUV-lamps can be used. The choice of the lamp depends on the absorptionspectrum of the radical former molecules used. Lamps having a higherpower generally permit more rapid cross-linking. The distance betweenthe UV-lamp(s) and the SAP which is to be cross-linked preferably variesbetween 5 cm and 15 cm.

Compared to the surface cross-linking known from the prior art, thesurface cross-linking according to the present invention is muchquicker. Prior art surface cross-linking reactions carried out underincreased temperatures commonly take up to 45 minutes. This timeconsuming process step renders the manufacturing process of SAPparticles less economic than desirable. On the contrary, thecross-linking process according to the present invention can be carriedout very quickly and hence, strongly adds to a much more efficient andeconomic overall manufacturing process.

Furthermore, as the surface cross-linking reaction proceeds quickly, themolecules comprised by the surface cross-linking composition applied onthe surface of the SAP particles have less time to penetrate inside theSAP particles. As a result, the surface cross-linking process is mainlyrestricted to the surface of the SAP particles and avoids undesiredfurther cross-linking reactions inside the SAP particles.

Another advantage of the present invention refers to the neutralizationstep. The α,β-unsaturated carboxylic acid monomers are often neutralizedprior to the polymerization step (pre-neutralization). Compounds, whichare useful to neutralize the acid groups of the monomers, are typicallythose, which will sufficiently neutralize the acid groups without havinga detrimental effect on the polymerization process. Such compoundsinclude alkali metal hydroxides, alkali metal carbonates andbicarbonates. Preferably, the material used for neutralization of themonomers is sodium or potassium hydroxide or carbonate. The neutralizingcompound is preferably added to an aqueous solution comprising theα,β-unsaturated carboxylic acid monomers (pre-neutralization). As aresult, the carboxyl groups comprised by the α,β-unsaturated carboxylicacid monomers are at least partially neutralized. Consequently, afterthe polymerization step- also the carboxyl groups comprised by theα,β-unsaturated carboxylic acid of the polymer are at least partiallyneutralized. In case sodium hydroxide is used, neutralization results insodium acrylate, which dissociates in water into negatively chargedacylate monomers and positively charged sodium ions.

If the final SAP particles are in the swollen state, after they absorbedaqueous solution, the sodium ions are freely movable within the SAPparticles. In absorbent articles, such as diapers or training pants, theSAP particles typically absorb urine. Compared to distilled water, urinecomprises a relatively high amount of salt, which at least partly ispresent in dissociated form. The dissociated salts comprised by theurine make absorption of liquid into the SAP particles more difficult,as the liquid has to be absorbed against an osmotic pressure caused bythe ions of the dissociated salts. The freely movable sodium ions withinthe SAP particles strongly facilitate the absorption of liquid into theparticles, because they reduce the osmotic pressure. Therefore, a highdegree of neutralization can largely increase the capacity of the SAPparticles and the speed of liquid absorption.

The surface cross-linkers known in the art react with the carboxylgroups of the polymer. Hence, the degree of neutralization has to bebalanced with the need to surface cross-link, because both process stepsmake use of the carboxyl groups.

According to the present invention, the surface cross-linkingcomposition comprises radiation activatable radical former molecules,which -once activated e.g. by UV radiation are able to react with thealiphatic groups comprised by the polymer. Therefore, it is possible toneutralize the monomers to a larger degree without significantlydiminishing the possibility of later surface cross-linking.

According to the present invention, the carboxyl groups comprised by theα,β-unsaturated carboxylic acid monomers are preferably at least 50%,more preferably at least 70%, even more preferably at least 75% and evenmore preferably between 75% and 95% neutralized. Hence, also thecarboxyl groups comprised by the α,β-unsaturated carboxylic acid of thepolymer are at least 50 %, more preferably at least 70%, even morepreferably at least 75% and even more preferably between 75% and 95%neutralized.

A still further advantage of the present invention is the reduction ofundesired side-reactions during the surface cross-linking process.Surface cross-linking known from the prior art requires increasedtemperatures, commonly around or above 150°. At these temperatures, notonly the surface cross-linking reaction is achieved, but also a numberof other reactions take place, e.g. anhydride-formation within thepolymer or dimer cleavage of dimers previously formed by the acrylicacid monomers. These side-reactions are highly undesired, because theyresult in SAP particles with decreases capacity.

As the surface cross-linking process according to the present inventiondoes not necessarily need increased temperatures but can also be carriedout at moderate temperatures using electromagnetic radiation, such as UVradiation, those side-reactions are considerably reduced. According tothe present invention, the surface cross-linking reaction can preferablybe accomplished at temperatures of less than 100° C., preferably attemperatures less than 80° C., more preferably at temperatures less than50° C., even more preferably at temperatures less than 40° C., mostpreferably at temperatures between 20° C. and 40° C. In an additionalprocess step drying of the SAP is typically carried out at temperaturesabove 100° C.

At elevated temperatures around or above 150° C. commonly applied in thesurface cross-linking process known from the prior art, the SAPparticles sometimes change their color from white to yellowish. Asaccording to the surface cross-linking process of the present invention,it is possible to carry out the surface cross-linking process undermoderate temperatures, the problem of color degradation of the SAPparticles is strongly reduced.

According to the present invention, the surface cross-linkingcomposition may comprise only one type of cross-linking molecules, ormay, alternatively, comprise two or more chemically differentcross-linking molecules. Likewise, the surface cross-linking compositionmay comprise only one type of radiation activatable radical formermolecule, or may, alternatively, comprise two or more chemicallydifferent radiation activatable radical former molecules.

As a further alternative, surface cross-linking composition of thepresent invention can be applied together with one or more thermallyactivatable surface cross-linkers, e.g. 1,4-butandiol. In thisembodiment, the SAP particles further have to comprise carboxyl groupswherein at least some of the carboxyl groups are at least partiallyexposed on the outer surface of the SAP particles and wherein thethermally activated surface cross-linker is covalently bound to at leasta part of the carboxyl groups at least partially exposed on the surfaceof said SAP particles.

In case the surface cross-linking composition of the present inventionis used together with a thermally activatable surface cross-linker, bothUV radiation and increased temperatures (above 140° C) are necessary forthe surface cross-linking process.

In these embodiments, the surface of the resulting SAP particles willfurther comprise the reaction product of the thermally activatablesurface cross-linker.

The surface cross-linking composition is preferably used in a liquidsolution, more preferably in an aqueous solution.

To obtain SAP particles with evenly distributed surface cross-linking,the surface cross-linking composition has to be distributed evenly onthe SAP particle prior to or during UV radiation. Therefore, the surfacecross-linker is preferably applied by spraying onto the SAP particles.

The method of the present invention may further comprise an optionalwashing step to wash off unreacted molecules comprised by the surfacecross-linking composition or to wash off molecules formed by sidereactions. Absorbent articles The SAP particles of the present inventionare preferably applied in absorbent articles. As used herein, absorbentarticle refers to devices that absorb and contain liquid, and morespecifically, refers to devices that are placed against or in proximityto the body of the wearer to absorb and contain the various exudatesdischarged from the body. Absorbent articles include but are not limitedto diapers, adult incontinent briefs, diaper holders and liners,sanitary napkins and the like.

Preferred absorbent articles of the present invention are diapers. Asused herein, “diaper” refers to an absorbent article generally worn byinfants and incontinent persons about the lower torso.

“Disposable” is used herein to describe articles that are generally notintended to be laundered or otherwise restored or reused i.e., they areintended to be discarded after a single use and, preferably, to berecycled, composted or otherwise disposed of in an environmentallycompatible manner.

FIG. 1 is a plan view of a diaper 20 as a preferred embodiment of anabsorbent article according to the present invention. The diaper isshown in its flat out, uncontracted state (i.e., without elastic inducedcontraction). Portions of the structure are cut away to more clearlyshow the underlying structure of the diaper 20. The portion of thediaper 20 that contacts a wearer is facing the viewer. The chassis 22 ofthe diaper 20 in FIG. 1 comprises the main body of the diaper 20. Thechassis 22 comprises an outer covering including a liquid pervioustopsheet 24 and/or a liquid impervious backsheet 26. The chassis 22 mayalso include most or all of the absorbent core 28 encased between thetopsheet 24 and the backsheet 26. The chassis 22 preferably furtherincludes side panels 30, leg cuffs 32 with elastic members 33 and awaist feature 34. The leg cuffs 32 and the waist feature 34 typicallycomprise elastic members. One end portion of the diaper is configured asthe front waist region 36 of the diaper 20. The opposite end portion isconfigured as the rear waist region 38 of the diaper 20. Theintermediate portion of the diaper is configured as the crotch region37, which extends longitudinally between the front and rear waistregions. The crotch region 37 is that portion of the diaper 20 which,when the diaper is worn, is generally positioned between the wearer'slegs.

The waist regions 36 and 38 may include a fastening system comprisingfastening members 40 preferably attached to the rear waist region 38 anda landing zone 42 attached to the front waist region 36.

The diaper 20 has a longitudinal axis 100 and a transverse axis 110. Theperiphery of the diaper 20 is defined by the outer edges of the diaper20 in which the longitudinal edges 44 run generally parallel to thelongitudinal axis 100 of the diaper 20 and the end edges 46 rungenerally parallel to the transverse axis 110 of the diaper 20.

The diaper may also include other features as are known in the artincluding front and rear ear panels, waist cap features, elastics andthe like to provide better fit, containment and aestheticcharacteristics.

The absorbent core 28 may comprise any absorbent material that isgenerally compressible, conformable, non-irritating to the wearer'sskin, and capable of absorbing and retaining liquids such as urine andother certain body exudates. The absorbent core 28 may comprise a widevariety of liquid-absorbent materials commonly used in disposablediapers and other absorbent articles such as comminute wood pulp, whichis generally referred to as air felt. Examples of other suitableabsorbent materials include creped cellulose wadding; melt blownpolymers, including co-form; chemically stiffened, modified orcross-linked cellulosic fibers; tissue, including tissue wraps andtissue laminates, absorbent foams, absorbent sponges, absorbent gellingmaterials, or any other known absorbent material or combinations ofmaterials. The absorbent core may further comprise minor amounts(typically less than 10%) of non-liquid absorbent materials, such asadhesives, waxes, oils and the like.

Furthermore, the SAP particles of the present invention can be appliedas absorbent materials. The SAP particles of the present inventionpreferably are present in amounts of at least 50% by weight of the wholeabsorbent core, more preferably at lest 60%, even more preferably atleast 75% and still even more preferably at least 90% by weight of thewhole absorbent core.

FIG. 2 shows a cross-sectional view of FIG. 1 taken in the transverseaxis 110. In FIG. 2 illustrates a preferred embodiment of the differentzones comprised by the absorbent cores. In FIG. 2, the fluid acquisitionzone 50 comprises an upper acquisition layer 52 and a lower acquisitionlayer 54, while the fluid storage zone underneath the fluid acquisitionzone comprises a storage layer 60, which is wrapped by an upper corewrap layer 56 and a lower core wrap layer 58.

In one preferred embodiment the upper acquisition layer comprises anonwoven fabric whereas the lower acquisition layer preferably comprisesa mixture of chemically stiffened, twisted and curled fibers, highsurface area fibers and thermoplastic binding fibers. In anotherpreferred embodiment both acquisition layers are provided from anon-woven material, which is preferably hydrophilic. The acquisitionlayer preferably is in direct contact with the storage layer.

In a preferred embodiment the core wrap material comprises a top layerand a bottom layer, which layers may be sealed together along theiredges, e.g. by adhesive. The top layer and the bottom layer can beprovided from a non-woven material. The top layer and the bottom layermay be provided from two or more separate sheets of materials or theymay be alternatively provided from a unitary sheet of material. Such aunitary sheet of material may be wrapped around the storage layer, e.g.in a C-fold.

The storage layer the present invention typically comprises SAPparticles mixed with fibrous materials. Other materials as suitable forthe absorbent core may also be comprised.

All documents cited in the Detailed Description of the Invention, are,in relevant part, incorporated herein by reference; the citation of anydocument is not to be construed as an admission that it is prior artwith respect to the present invention. To the extent that any definitionor meaning of a term in this written document conflicts with anydefinition or meaning of the term in a document incorporated byreference, the definition or meaning assigned to the term in thisdocument shall govern.

While particular embodiments of the present invention have beenillustrated and described, it would be obvious to those skilled in theart that various other changes and modifications can be made withoutdeparting from the spirit and scope of the invention. It is thereforeintended to cover in the appended claims all such changes andmodifications that are within the scope of this invention.

1. A superabsorbent polymer, comprising: a. particles selected from thegroup consisting of granules, flakes, spheres, powders, platelets, andbeads, each of the particles comprising a surface and a core, wherein aplurality of aliphatic C—H groups are present at the surface; and b.polymer chain segments, wherein at least some of said polymer chainsegments are covalently surface cross-linked to each other afterformation of said superabsorbent polymer particles, wherein at leastsome of the surface cross-links comprise the reaction product ofcross-linking molecules having at least two C═C double bonds, at leastsome of the cross-linking molecules comprising a water soluble peroxideradical initiator, wherein at least some of the surface cross-linkscomprise the reaction product of radical former molecules thatchemically react with the aliphatic C—H groups when the radical formermolecules are activated.
 2. Superabsorbent polymer particles accordingto claim 1, wherein said radical former molecules are radical formermolecules of the photo-reduction type.
 3. Superabsorbent polymerparticles according to claim 1, wherein said superabsorbent polymerparticles comprise the reaction products of at least two chemicallydifferent cross-linking molecules.
 4. Superabsorbent polymer particlesaccording to claim 1, wherein said superabsorbent polymer particlescomprise the reaction products and of at least two chemically differentradical former molecules.
 5. A method of surface cross-linkingsuperabsorbent polymer particles comprising: providing discretesuperabsorbent polymer particles selected from the group consisting ofgranules, flakes, spheres, powders, platelets, and beads, each of theparticles comprising a surface and a core, wherein a plurality ofaliphatic C—H groups are present at the surface, the particlescomprising polymer chain segments; adding a surface cross-linkingcomposition comprising cross-linking molecules having at least two C═Cdouble bonds and further comprising radical former molecules wherein theradical former molecules when activated are configured to chemicallyreact with the aliphatic C—H groups; and exposing said superabsorbentpolymer particles and said surface cross-linking composition toelectromagnetic irradiation capable of activating said radical former,whereby said cross-linking molecules and said radical former moleculesreact with at least some of said polymer chain segments comprised atsurfaces of the superabsorbent polymer particles to form covalentcross-links between said polymer chain segments, wherein saidcross-links comprise the reaction product of said cross-linkingmolecule, wherein said cross-links further comprise the reaction productof said radical former molecules, and wherein the method does not leadto any appreciable interparticulate bonds between the superabsorbentpolymer particles.
 6. The method according to claim 5, wherein saidelectromagnetic irradiation is UV radiation.
 7. The method according toclaim 5, wherein said radical former molecules are radical formermolecules of the photo-reduction type.
 8. The method according to claim5, wherein said cross-linking composition comprises at least twochemically different cross-linking molecules.
 9. The method according toclaim 5, wherein said cross-linking composition comprises at least twochemically different radical former molecules.
 10. The method accordingto claim 5, wherein said method is carried out at temperatures of lessthan about 100° C.
 11. The method according to claim 5, wherein saidsurface cross-linking composition further comprises a solvent andwherein said method further comprises the step of drying saidsuperabsorbent polymer particles, said drying being carried out afterstep c) of claim
 5. 12. An absorbent article comprising a substantiallyliquid pervious topsheet, a substantially liquid impervious backsheetand an absorbent core between said topsheet and said backsheet, whereinsaid absorbent article comprises superabsorbent polymer particlesaccording to claim
 1. 13. An absorbent article comprising superabsorbentpolymer particles, said superabsorbent polymer particles being madeaccording to a process of claim 5.